1. Field of the Invention
The present invention relates to an image forming apparatus having a plurality of developing devices in correspondence to one image bearing member, in which the plurality of developing devices are switched in a method of executing development by causing the developing devices to sequentially approach the image bearing member in confrontation therewith.
2. Description of the Related Art
Heretofore, when a color image is formed, there is employed a method of developing an electrostatic image formed in response to external information using developer of a plurality of colors, sequentially forming developing agent images of the plurality of colors (toner images) on a photosensitive drum acting as an image bearing member, and overlapping the toner images of the plurality of colors on a transferring material such as a sheet or the like sequentially or at a time.
In color image forming apparatuses arranged as described above, there has been proposed and in practical use a so-called rotary development system. The rotary development system employs a rotary type developing apparatus that includes developing devices for developers (toner) of respective colors, for example, black, yellow, magenta, and cyan, mounted on rotating means (rotary), which is free to rotate, around the rotating periphery thereof and executes a developing operation by sequentially moving a necessary developing device to a developing portion that confronts the photosensitive drum acting as the image bearing member by rotating the rotary.
Incidentally, in the conventional image forming apparatus having the rotary developing apparatus arranged as described above, the respective developing devices mounted on a rotary reach a developing portion from an image forming apparatus main body in development, and a developing voltage is applied to them at the position of the developing portion. Accordingly, an electric connecting portion, that is, an image forming apparatus side connecting portion (apparatus main body side connecting portion) acting as a power supply connecting portion is disposed externally of the developing devices. In contrast, the developing devices are provided with developing device side connecting portions acting as power-to-be supplied connecting portions which are electrically connected to the apparatus main body side connecting portion. With this arrangement, when the developing devices reach the developing portion, the apparatus main body side connecting portion is connected to the developing device side connecting portions. Refer to, for example, Japanese Patent Application Laid-Open No. 2001-281950.
An image forming apparatus having three sets of developing devices 5 mounted on a rotary 8 as shown in, for example, FIG. 13 will be explained as an apparatus for realizing the above arrangement. The image forming apparatus is generally arranged as described below. That is, the developing devices 5 have protruding connecting portions 51 which protrude from the main bodies thereof as shown in FIG. 14. The main body of the image forming apparatus has a plate-shaped (or sheet-shaped) connecting portion 81 acting an apparatus main body side connecting portion. The connecting portions 81 is disposed at a position confronting the connecting portion 51 of a developing device 5 located at a developing portion P. With the above arrangement, when the rotary 8 rotates and the developing device 5 reaches the developing portion P, the protruding connecting portions 51 of the developing device 5 is abutted against the connecting portions 81 of rotary 8 and electrically connected thereto. There may be provided a plurality of positions as a contact position 58 at which the developing device side protruding connecting portions (developing device side connecting portion) 51 comes into contact with the apparatus main body side plate-shaped connecting portions (apparatus main body side connecting portions) 81. In FIG. 13, two contact positions 58 are provided for each of developing devices 5.
In this arrangement, the plate-shaped connecting portion 81 is provided with a spring property and secured in an image forming apparatus main body, for example, it is disposed to a cabinet by which the rotary 8 is supported. In the image forming apparatus of FIG. 13, bearings 8a, which are disposed on both the sides the rotating shaft of the rotary 8, are composed of plate-shaped (or sheet-shaped) members extending from the rotating shaft of the rotary 8. The plate-shaped members are disposed on both the sides of the developing devices 5 in the vicinity thereof. The conductive connecting portion 81 is attached to one of the bearing plate (or sheet) 8a by being, for example, bonded thereto or embedded therein. The connecting portion 81 is disposed at a portion confronting one side of a developing device 5 located at the developing portion P with its conducting portion facing the developing device 5.
As shown in FIG. 14, the protruding connecting portion 51 of each developing device 5 protrudes from the one side thereof, and the conducting portion at the extreme end the connecting portion 51 comes into contact with the connecting portion 81, which is disposed to one of the bearing plates 8a adjacent to both the sides of the developing device 5 in confrontation therewith, at the developing portion P.
As described above, the connecting portions 51 of the developing devices 5 are abutted against the connecting portion 81 disposed to the bearing plate 8a of the rotary 8 at the developing portion P. When the developing devices 5 are switched by rotating the rotary 8, the connecting portions 51 are in sliding contact with the connecting portion 81. Accordingly, in an image forming apparatus that can form an image at high speed, the rotating speed of the rotary 8 is also increased, and thus is difficult to guarantee the durability of the connecting portions 51 and 81.
As a countermeasure for the above drawback, many examples reduce the moving speed of the connecting portions 51 on the developing device 5 side by disposing the contact positions 58 of the developing device side connecting portions 51 and the apparatus main body side connecting portion 81 at positions as nearer to the center of rotation X of the rotary 8 as possible as shown in FIG. 13.
Further, springs 82 are interposed between the apparatus main body side connecting portions 81 and the bearing plate 8a as shown in FIG. 14 to maintain a state that the connecting portion 51 is securely abutted against the connecting portion 81 at the contact positions 58, thereby the connecting portion 81 is pressed against the connecting portion 51.
Note that, the connecting portions 51 and 81 are disposed in the image forming apparatus main body such that the conductive portions thereof do not come into contact with the members other than them so as to prevent an electric leakage.
In the arrangement of the conventional connecting portion described above, however, since the image forming speed is in a contradictory relationship to the durability of the connecting portions 51 and 81, an increase in the rotating speed of the rotary 8 increases a speed at which the connecting portion 51 is in sliding contact with the connecting portion 81, from which an unavoidable problem arises in grinding and breakage of the connecting portions 51 and 81.
Further, when the connecting portion 51 is abutted against the connecting portions 81, the connecting portion 51 is slightly separated from the connecting portions 81 inevitably by the repulsion caused when they collide against each other. Thus, the electric leakage may occur when a developing voltage is applied at the time. In an image forming apparatus in which the rotating speed of the rotary 8 is relatively slow, an effective countermeasure can be relatively simply employed to overcome the above drawback by increasing the force of the springs 82 of the apparatus main body side connecting portion disposed externally of the rotary 8. In an image forming apparatus, in which the rotating speed of the rotary 8 is 150 mm/sec in the vicinity of the contact positions 58, however, a more severe sliding condition is imposed on the connecting portions 51 and 81, from which problems relating to the durability thereof arise inevitably in many cases.
Further, restrictions are naturally imposed also on an arrangement in which an image is formed by reducing the moving speed of the developing device side connecting portions 51 by disposing the contact positions 58 in the vicinity of the center X of the rotary 8 to improve the sliding condition of the connecting portions described above. Since the area of the abutting surface of the apparatus main body side connecting portion 81 is reduced with respect to, for example, a necessary movement angle, it is difficult for the connecting portion 81 to maintain a stable abutting state against the developing device side connecting portion 51 in the above arrangement.